Basically just "Set as default recipe" but able to do for current machine category. Doesn't need to be visible button/can be unbound by default; I'd just like to not have to click the heart 600+ times per machine in GT/etc.

Doesn't need to be more complex than iterating over all recipes in that category view as if one cycled through and manually clicked set as default on every single one.

I do recognize the fact that a specific output may have more than one recipe in the same category, and it doesn't matter. Whichever ends up set can be random/last iterated.
(Gregtech example w/ Bender machine: Iron ingot+circuit2->"double plate", but iron plate+circuit2->"double plate" as well, either work and one is more desirable than the other; but either is more desirable than the other categories alternatives. Any undesired conflicts can be resolved manually later as normal, but they would have had to be manually set anyway so it's still less or same effort.)

Mostly a power-user feature, doesn't need to be any kind of foolproof. Could easily have a tooltip saying "Set all recipes in this category as default, with recipe output conflicts deciding randomly."

If anything for an extension to this, maybe a 2nd bind for "set any recipe outputs that don't already have a default set in this category", doing the same thing but if that category already has an output defined as default then don't change it....
Still easy to iterate with, just take an index of the existing set-defaults in that category, run the first randomly-setting operation, then overwrite/iterate with the indexed list. Scuffed? Maybe a bit, but very useful.
So with the "Bender machine category" example, if I had already set iron_ingot->iron_double_plate to default, it wouldn't potentially get overwritten with iron_plate->iron_double_plate (rather, it would get overwritten then reverted). Meanwhile the "Crafting table category" hammer+copper_ingot*2->copper_double_plate that's still set to default would get replaced with bender:copper_ingot->copper_double_plate in the "bender machine category".